“Being able to effectively manage our instrument configurations
reduces our regulatory risk and our risk to product,” says Jerry
Lowery of The Ohio State University (USA) as he attests to the essential
value of HART Communication in industrial automation applications.
One benefit he names is that most technicians are familiar with HART-enabled
communicators.
“The technicians just love them. They’re superior for
the technicians,” he says. Another important feature for Lowery,
who is a control systems engineer for the university’s McCracken
power plant, is that HART is an open standard. “It gives everyone
a common thing to work toward. HART is the way to go. It makes life
so much easier.”
Operating continuously, the McCracken plant provides 85% of the Buckeye
campus’ steam-based energy needs. The steam is used for heating,
humidification, sterilization, chilled-water, and domestic hot-water
production.
The plant has five industrial boilers, a Babcock & Wilcox D-Style,
600-psi, 220,000 lbs/hr unit, and four 200-psi Nebraska D-style units
that produce 150,000 lb/hr of steam,. Co-located at the plant are
ten York chillers producing 16,000 TR/hr, and three Caterpillar emergency
diesel generators with a rated total of 6,300 kWH.
Lowery doesn’t mince words about how important HART is to the
plant. “We have approximately 250 HART-enabled devices. Without
HART, this plant wouldn’t operate,” he says. HART also
helps the plant’s technicians save time finding instrument problems.
“We don’t have to spend as much time looking for what’s
wrong,” Lowery adds.
In one instance, staffers observed intermittent signal failures on
a temperature transmitter on one boiler’s superheater (SH) vent
control, which caused the vent to go into the wrong position. However,
thanks to a HART-enabled Honeywell DCS system on the Nebraska boiler,
they were able to quickly correct the problem.
Intermittent signal failures are some of the more difficult field
problems to diagnose as the problem may only happen occasionally for
no apparent reason. ”Before having continuous HART monitoring
of devices, we would have to guess at the problem if it wasn’t
apparent and start replacing items (sensor, transmitter, or analog
input) to see if the problem would go away, says Lowery. “With
HART and the ability to continuously monitor the HART signal from
the device, we are able to log the issue in the DCS/Asset Manager
software and go out and correct the actual problem.
“We have two quad operator stations with four human-machine
interface (HMI) screens and four others where HART data appears via
the DCS,” he explains. “This was the first time we’d
seen this type of HART information.”
The displayed spreadsheet showed data which has a log/time-stamp describing
an event. In this case, descriptions included “primary variable
(PV) was out of sensor limits” or “bad: check measuring”
and “field device malfunction.”
Once the staff learned of the event descriptions in the event log,
they were able to solve their problem the same day. “We knew
exactly what was wrong. The technician went out and repaired the resistance
temperature device (RTD),” Lowery says, adding that with their
HART-enabled DCS, “our technicians know exactly what the problem
is.” Results Proven, Risks Lessened
A second occurrence at Ohio State involved an Superheater (SH) vent-control
valves failure. The boiler superheater vent valve wouldn’t
open or would become stuck while partially open, Lowery recalls.
“A vent-control valve failure put the boiler superheater at
risk.”
Lowery’s solution was to use the HART Protocol’s digital
PV (DigPV) that was accessible from the DCS. He used the valve positioner
digital PV to validate the valve’s position. He says if the
valve didn’t reach proper position within 30 seconds, an alarm
would sound and a warning would simultaneously be displayed on the
HMI screen.
The DigPV, which proved that the valve was not seating properly,
and the digital secondary variable (DigSV), in this case positioner
temperature, indicated that a temperature “exceedance”
had occurred.
“The HART secondary value or device temperature on the original
positioner was trended and used to prove that the device had never
exceeded its maximum operating temperature of 50 ºC,”
says Lowery.
The data convinced the vendor to replace the valve positioner, “and
the problem went away.” This is important because the vendor
originally claimed the boiler’s operation had overheated and
caused the SH vent-control valve to become stuck.
Armed with their HART-enabled DCS evidence, Ohio State got the vendor
to fix six SH vent valves saving approximately $10,000 (USD). Also,
the combined digital PVs’ and SVs’ performance not only
saved a potential $300,000 (USD) cost of replacing the existing
superheater, but also led the vendor to install a more reliable
positioner.
Lowery appreciates this digital value functionality. “Digital
PVs are extremely valuable in critical loops to validate the analog
position with the digital position,” he says. In addition
to diagnostics, having both analog and digital communication available
simultaneously with HART is of particular value to Lowery.
“This capability is extremely important when validating loops.
We use the digital PVs to validate analog PVs on critical positioners.
It just tells us what is wrong with the device,” he states.
“And on critical loops, if the analog PVs and digital PVs
don’t match up within a certain percentage of each other,
we alarm that loop and have operators investigate the actual valve
position.”
The simultaneous analog-digital capability of HART fulfills one
of the most obvious asset management functions of HART-enabled automation—protecting
equipment and keeping plants operating.
Of the 250 HART-enabled devices at the McCracken plant, an asset
management system (AMS) monitors 188-200 of them. With a separate
server, the AMS has a processor and input/output (I/O) modules.
“The asset manager monitors and tracks device faults for all
HART-enabled devices. Types of device faults —anything you
can think of,” he says.
Overall, diagnostics are much improved. “Our asset manager
package tracks faults that occur in HART devices through our DCS.
Small items, like a positioner losing its zero position, are now
caught early and repaired,” Lowery says. “But without
that asset management software, the problem might have never been
corrected because it wouldn’t have been seen until complete
device failure.”
Correct monitoring, particularly with HART-enabled instruments,
also concerns Lowery, who offers tips for users. “HART device
diagnostics is only as good as the vendor’s device description
(DD) file. The more detailed the DD file, the more diagnostics you’ll
have.”
Lowery also suggests insuring that the DCS analog I/O is HART-enabled.
“Using a third-party solution to strip out and send HART data
via a serial connection will likely prevent using HART digital PVs
in a fast changing loop,” he says. Caveat emptor (Let the
buyer beware!) is his advice regarding proprietary functions in
HART devices.
“Some vendors tweak a HART DD file to their advantage, so
you’ll use their DCS with their HART devices,” Lowery
says. He advises users to make sure all functions are accessible.
“Check that the DD file registered at the HART Communication
Foundation is the file that the vendor distributes. Also, consider
joining the Foundation. DD files from the Foundation are tested
to be sure they are compliant to the standards.”
Ohio State’s experience with HART was so positive that, according
to Lowery, the university has only scratched the surface of using
the protocol. For quality assurance, the power plant’s staff
is examining tracking the drift of the analog PV compared to the
digital PV. Also, there’ll be more use of HART device diagnostic
faults in the control strategy. Adding this tracking ensures agreement
between the digital PV and analog PV readings.
Ohio State also plans to evaluate using a field device manager (FDM)
for configuring HART devices, rather than using HART communicators.
How will that affect operations? “It will help us catalog
devices. We’re told that if you replace a device, when you
plug it in, the DCS will automatically load its tag and other operating
information,” Lowery says.
Finally, through its asset manager, the university will increase
fault modeling at McCracken. What part will HART play? “All
HART devices can tell you if there’s a fault,” Lowery
says. “We can go to redundant format automatically.”
That’s important, he notes, because the analog PV value could
be in error.
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